Enhancement: Stream JSON output for ls command

The `ls` command now supports JSON output with the global `--json`
flag, and this change streams out JSON messages one object at a time
rather than en entire array buffered in memory before encoding. The
advantage is it allows large listings to be handled efficiently.

Two message types are printed: snapshots and nodes. A snapshot
object will precede node objects which belong to that snapshot.
The `struct_type` field can be used to determine which kind of
message an object is.

https://github.com/restic/restic/pull/1962
